When one carries out the hydrogenation of adiponitrile (ADN) to produce the fully hydrogenated product, hexamtheylenediamine (HMD), or the partially hydrogenated product, epsilon-aminocapronitrile (ACN), one inevitably produces some byproducts, including certain imines. Imines are problematic in further processing the HMD and ACN to Nylon 6,6 and Nylon 6, respectively, because the imines impart an undesirable color to the nylons, and the imines may limit the molecular weight that the nylon polymers can attain. (Generally, high molecular weights are preferred for nylon.) The most common problematic imine in ADN hydrogenation is tetrahydroazepine (THA). One can use a gas chromatograph to measure THA, or a polarograph to measure all of the so-called polarographically reducible impurities (PRI) in the measured sample, the major component of the PRI being THA. The polarographic measurement generally will be somewhat higher than the chromatographic measurement, because a polarograph measures polarographically reducible impurities other than THA.
Conceptually, one may envisage the making of HMD by allowing ADN molecules to move slowly through a hydrogenation reactor with a low space velocity, providing plenty of time for hydrogen to react with ADN to convert both of the ADN nitrile groups to amine groups. In contrast, if ADN is allowed to move quickly through the hydrogenation reactor at a higher space velocity, thereby shortening the time that the hydrogen can react with the ADN, the crude reaction product will contain the fully hydrogenated product, HMD, the partially hydrogenated product, ACN, as well as unreacted ADN. Operating in this latter way, generally referred to as “partial hydrogenation,” would allow one hydrogenation reactor to be used to prepare intermediates for two types of nylons: ACN for Nylon 6 and HMD for Nylon 6,6. It is known that when one does partial hydrogenation, the PRI (mainly THA) levels will be higher than if one does complete hydrogenation. With the increased reaction time inherent in complete hydrogenation, the THA can react with the hydrogen to convert the carbon-nitrogen double bond of the THA into a carbon-nitrogen single bond, leading to the formation of a product called hexamethyleneimine (HMI), perhaps confusingly named, because its molecular structure does not meet one definition of an imine, namely the presence of a carbon-nitrogen double bond.
A necessary part of a partial hydrogenation process is the removal of PRI (mainly THA) from the crude ADN hydrogenation product before polymerization. Different approaches for accomplishing this have been disclosed in the art, including various distillations that can separately recover unreacted ADN, ACN and HMD. These distillations, however, tend to leave the PRI (mainly THA) in the ACN fraction that results from the distillation. Some investigators have proposed hydrogenating the PRI in the ACN fraction (see U.S. Pat. No. 6,1537,48), but this approach creates the potential of further hydrogenating the ACN to HMD, thus reducing the yield of the desirable ACN, and necessitating further distillations to separate the ACN from the HMD. Others have suggested separation by electrochemical reduction.
A commercially viable partial hydrogenation process must be capable of producing HMD that contains low levels of THA, generally less than 200 ppm, preferably less than 100 ppm.